From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6ED4DC352AA for ; Mon, 7 Oct 2019 11:33:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 313AF20867 for ; Mon, 7 Oct 2019 11:33:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1570447985; bh=xArUum528Sfkwj0dI3uMICujB7KB2afd3HvkaXtrqnI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=Q4G8Z3bOirYs+Su/SSimDQG2CSLq6qsg6qK13jEVW0yHA7mrOFDxhQRlJtqhBT6VF gmrAl3d1xq8Bxe+y/0YNKwooKKEJVUIh9hR1GjAf/I/o8rUNsbIWLz9mYDtBbJ660z uncVUG/PcaaB0dVPQP0Qm3LsQ4CPv8rcpTyUSD8Y= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727608AbfJGLdE (ORCPT ); Mon, 7 Oct 2019 07:33:04 -0400 Received: from mx2.suse.de ([195.135.220.15]:45764 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727467AbfJGLdE (ORCPT ); Mon, 7 Oct 2019 07:33:04 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 4BCF5ABD9; Mon, 7 Oct 2019 11:33:02 +0000 (UTC) Date: Mon, 7 Oct 2019 13:33:01 +0200 From: Michal Hocko To: Petr Mladek Cc: Qian Cai , akpm@linux-foundation.org, sergey.senozhatsky.work@gmail.com, rostedt@goodmis.org, peterz@infradead.org, david@redhat.com, john.ogness@linutronix.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk() Message-ID: <20191007113301.GG2381@dhcp22.suse.cz> References: <1570228005-24979-1-git-send-email-cai@lca.pw> <20191007080742.GD2381@dhcp22.suse.cz> <20191007090553.g5cq7qa4tj5yrtaa@pathway.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191007090553.g5cq7qa4tj5yrtaa@pathway.suse.cz> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 07-10-19 11:05:53, Petr Mladek wrote: > On Mon 2019-10-07 10:07:42, Michal Hocko wrote: > > On Fri 04-10-19 18:26:45, Qian Cai wrote: > > > It is unsafe to call printk() while zone->lock was held, i.e., > > > > > > zone->lock --> console_lock > > > > > > because the console could always allocate some memory in different code > > > paths and form locking chains in an opposite order, > > > > > > console_lock --> * --> zone->lock > > > > > > As the result, it triggers lockdep splats like below and in different > > > code paths in this thread [1]. Since has_unmovable_pages() was only used > > > in set_migratetype_isolate() and is_pageblock_removable_nolock(). Only > > > the former will set the REPORT_FAILURE flag which will call printk(). > > > Hence, unlock the zone->lock just before the dump_page() there where > > > when has_unmovable_pages() returns true, there is no need to hold the > > > lock anyway in the rest of set_migratetype_isolate(). > > > > > > While at it, remove a problematic printk() in __offline_isolated_pages() > > > only for debugging as well which will always disable lockdep on debug > > > kernels. > > > > I do not think that removing the printk is the right long term solution. > > While I do agree that removing the debugging printk __offline_isolated_pages > > does make sense because it is essentially of a very limited use, this > > doesn't really solve the underlying problem. There are likely other > > printks from zone->lock. It would be much more saner to actually > > disallow consoles to allocate any memory while printk is called from an > > atomic context. > > The current "standard" solution for these situations is to replace > the problematic printk() with printk_deferred(). It would deffer > the console handling. > > Of course, this is a whack a mole approach. The long term solution > is to deffer printk() by default. We have finally agreed on this > few weeks ago on Plumbers conference. It is going to be added > together with fully lockless log buffer hopefully soon. It will > be part of upstreaming Real-Time related code. OK, then we do not really have to do anything here. That is good to hear because I really detest putting printk_deferred or anything like that at random places. -- Michal Hocko SUSE Labs